This disclosure pertains generally to microlithography performed using a charged particle beam such as an electron beam or ion beam. Microlithography is a key technology used extensively in the fabrication of microelectronic devices such as integrated circuits, displays, thin-film magnetic-pickup heads, and micromachines. In charged-particle-beam (CPB) microlithography the pattern to be transferred to a substrate is typically defined on a segmented reticle, i.e., a reticle that is divided into multiple exposure units (termed xe2x80x9csubfieldsxe2x80x9d). Each subfield defines a respective portion of the pattern and is exposed individually. More specifically, the disclosure pertains to, inter alia, methods and apparatus for evaluating the beam blur in a CPB microlithography system.
Conventional charged-particle-beam (CPB) microlithography methods (typically using an electron beam) suffer from the disadvantage of low throughput (i.e., number of production units such as wafers that can be processed per unit time). Substantial research and development is being directed to improving the throughput so as to provide a CPB microlithography technology that is practical for mass-production of microelectronic devices. For example, electron-beam xe2x80x9cdirect-drawingxe2x80x9d techniques currently are used mainly for manufacturing reticles, and these techniques can be used for forming patterns directly on the surface of a suitable substrate. However, with direct-drawing methods, the pattern is formed on the substrate line-by-line or feature-by-feature, which requires enormous amounts of time per wafer. Hence, there are severe limits to the maximum throughput currently achievable using conventional electron-beam direct-drawing methods.
Various approaches to CPB microlithography have been considered in an effort to increase throughput. CPB-microlithographic exposure of an entire pattern (defined on a reticle) to the substrate in one exposure xe2x80x9cshot,xe2x80x9d in the manner used in optical microlithography, would appear to be the best solution. However, this approach has to date been impossible for various reasons. One promising approach offering prospects of acceptable throughput is termed xe2x80x9cdivided-reticlexe2x80x9d CPB microlithography, in which the pattern as defined on the reticle is divided into a large number of xe2x80x9csubfieldsxe2x80x9d each defining a respective portion of the pattern. Although each subfield defines only a portion of the pattern, the respective pattern portions are substantially larger than single features. Consequently, throughput is higher than with direct-drawing methods (but not as high as would be obtained if the entire reticle were exposed in one shot).
With the divided-reticle approach, achieving higher throughput principally involves configuring the subfields as large as possible so that the pattern portion projected with each respective shot is as large as possible. Increasing subfield size requires corresponding increases in the numerical aperture of the CPB optical system, which correspondingly increases the difficulty of correcting off-axis aberrations and beam blur. Hence, divided-reticle microlithography systems currently under development include respective subsystems for measuring aberrations and beam blur and for performing adjustments of the beam as required to correct the aberrations and beam blur. These adjustments typically include corrections of focal point, astigmatism, magnification, image rotation, and other parameters that impact the imaging performance of the system.
FIG. 13 is an oblique view schematically illustrating a conventional method for measuring beam blur; FIG. 14 depicts certain details of the method in schematic block form; and FIG. 15 is a plot of exemplary measurement results obtained using the method. This method for measuring beam blur is disclosed, for example, in Japan Kxc3x4kai Patent Document No. Hei 10-289851 (corresponding to U.S. Pat. No. 6,059,981) and in Japan Kxc3x4kai Patent Document No. 2001-203149.
Referring first to FIG. 13, it will be understood that an illumination-beam source and a reticle, although not shown, are located upstream of the components shown in the figure. The reticle, in addition to defining the pattern to be transferred to the substrate, also defines a pattern of measurement marks. The beamlet EB depicted in FIG. 13 is the small electron beam produced by transmission of the illumination beam through a measurement mark on the reticle. Hence, the beamlet EB that has passed through the measurement mark has a rectangular transverse profile. The measurement mark typically is rectangular in profile. The beamlet EB is incident on a plate 100 that defines a xe2x80x9cknife-edgedxe2x80x9d reference mark 102. The reference mark 102 typically is rectangular in profile (the entire mark is not shown) and is configured as a respective through-hole defined by the plate 100 (see FIG. 14). The beamlet EB is incident in a scanning manner on a knife-edge 101 of the mark 102, wherein the knife-edge 101 is configured as in the well-known xe2x80x9cknife-edgexe2x80x9d test for evaluating the quality of an aerial image. An electron detector (sensor) 105 is disposed downstream of the mark 102.
As the beamlet EB is scanned in a direction indicated by a respective arrow (labeled xe2x80x9cSCANxe2x80x9d; e.g., extending to the right in FIGS. 13 and 14), electrons of the beamlet EB incident on the plate 100 itself either are absorbed (if the plate 100 has sufficient thickness) or transmitted with forward-scattering (if the plate 100 is sufficiently thin). On the other hand, electrons incident on the reference mark 102 are transmitted through the reference mark and are detected by the electron detector 105. As noted above, if the plate 100 is sufficiently thin (e.g., made of silicon having a thickness of 2 xcexcm), nearly all the electrons incident on the plate 100 are transmitted with forward-scattering through the plate. (Conventionally, configuring the plate 100 sufficiently thin so as to cause forward-scattering of incident electrons is preferred. Also, a thinner plate provides a greater geometric accuracy of the knife-edged reference mark 102.) In the following discussion, it is assumed that the plate 100 is sufficiently thin to cause forward-scattering.
In view of the above, the electrons detected by the electron detector 105 include non-scattered electrons e1 (that have passed directly through the reference mark 102) and electrons e2 (that were forward-scattered during transmission through the plate 100). The respective beam currents for the electrons e1 and e2, as detected by the detector 105, are amplified by a pre-amplifier 106, converted by a differentiation circuit 107 (wherein the conversion is to percent change versus time) to an output waveform, and displayed on an oscilloscope 108 or analogous display. Beam blur is determined from the output waveform produced by the differentiation circuit 107. From the determined beam blur, appropriate corrective adjustments (e.g., of focal point, astigmatism, magnification, rotation, etc.) are made to the beam. After making the corrective adjustments, another beam-blur measurement can be made to ascertain whether the corrective adjustments were appropriate.
In this conventional beam-blur measurement method, contrast of the xe2x80x9cimagexe2x80x9d detected by the detector 105 is a function of the difference in electron scattering produced by the reference mark 102 versus the electron scattering produced by the plate 100. Unfortunately, most of the forward-scattered electrons e2 propagate to the detector 105. These detected forward-scattered electrons e2 reduce measurement contrast. Specifically, the forward-scattered electrons e2 are a source of measurement noise, as shown in FIG. 15. The noise produces an actual detected-current waveform Wxe2x80x2 that is offset from an ideal detected-current waveform W (based at the xe2x80x9c0xe2x80x9d level). Also, leading edges of the detected waveform have a gradual slope, which results in decreased measurement accuracy.
Modern divided-reticle CPB microlithography apparatus are configured to expose individual subfields measuring 250 xcexcm square, for example, which is quite large. Simulation studies have revealed that beam blur over such an area exhibits a distribution in which the magnitude of blur is a function of location within the subfield. Simulation studies also have revealed that, whenever the current of an electron beam is increased so as to increase throughput, space-charge effects produce a distribution of beam blur within individual subfields. As a result, it is necessary to measure the distribution of beam blur with extremely high accuracy and precision. Such measurements simply are not obtainable using conventional methods, even in conventional methods in which beam blur is measured at a single point.
In view of the deficiencies of conventional methods and devices as summarized above, the present invention provides, inter alia, methods and devices for evaluating imaging performance (specifically beam blur) of a charged-particle-beam (CPB) microlithography apparatus. The distribution of beam blur can be measured at high accuracy at one or more locations within a subfield.
A first aspect of the invention is set forth in the context of a CPB microlithography method. In the microlithography method a reticle, defining a pattern to be transferred to a sensitive substrate, is irradiated with a charged-particle illumination beam, and a charged-particle patterned beam, formed by passage of the illumination beam through an illuminated portion of the reticle and carrying an aerial image of the illuminated portion of the reticle, is projected onto a sensitive surface of a substrate. Thus, the sensitive surface is imprinted with the aerial image. In this context, methods are provided for evaluating the lithographic imaging performance. An embodiment of such a method comprises defining a beam-transmitting measurement mark at an object plane and defining a knife-edged reference mark, at an image plane, as a corresponding through-hole in a charged-particle-scattering membrane. The measurement mark is illuminated with a charged particle beam to form a charged-particle beamlet propagating downstream of the measurement mark toward the reference mark. The beamlet is projected onto the reference mark while scanning the beamlet over a knife-edge of the reference mark to produce non-scattered charged particles transmitted through the through-hole and forward-scattered charged particles transmitted through the membrane. The non-scattered and forward-scattered charged particles propagate downstream of the reference mark. A beam-limiting diaphragm is disposed downstream of the reference mark. The beam-limiting diaphragm comprises a diaphragm plate defining a beam-limiting aperture having a diameter sufficient to block most of the forward-scattered charged particles while not blocking the non-scattered charged particles from reaching the detector. The beam current of charged particles propagating downstream of the beam-limiting diaphragm is measured.
Because most of the beamlet passing through the membrane (defining the knife-edged reference mark) is blocked, substantially only the non-scattered charged particles of the beamlet passing through the reference mark are incident on the detector. By eliminating most of the forward-scattered charged particles, measurement noise is substantially reduced compared to conventional methods, without any adverse effect on measurement contrast. Furthermore, in situations in which a dummy beam is used to adjust beam current (to control space-charge effects), most of the forward-scattered charged particles from the dummy beam can be blocked from reaching the detector. This allows a good detection waveform to be obtained with nearly ideal contrast.
The beamlet normally is projected using first and second projection lenses. In such a situation, the axial distance from the knife-edged reference mark to the beam-limiting diaphragm is such that an axial angle (as measured at the knife-edge) subtended by the beam-limiting aperture is slightly greater than a beam-convergence angle (or half-aspect angle) of the beamlet at the substrate. One desirable range for xe2x80x9cslightly greaterxe2x80x9d is 1.1 to 3 times the angle of convergence angle. For example, in a situation in which a dummy beam is used, if the diameter of the beam-limiting aperture is 50 xcexcm or less (desirably about 10 xcexcm), the axial angle (as measured at the knife-edge) subtended by the beam-limiting aperture is 10 mrad, the beam-convergence angle of the beamlet is 5 mrad, and the axial distance from the reference mark to the beam-limiting diaphragm is about 2.5 mm. In this example, nearly all the dummy beam transmitted through the membrane defining the knife-edged reference mark is blocked by the beam-limiting aperture plate, allowing beam-blur measurements to be obtained at full contrast.
The step of defining a measurement mark can comprise defining multiple beam-transmitting measurement marks in a subfield of a reticle disposed at the object plane. In this instance, the detecting step comprises detecting a distribution of beam blur within the subfield.
The method can further comprise defining a dummy pattern around the measurement mark, as defined in a subfield of the reticle disposed at the object plane. In this instance, as the measurement mark is illuminated with the charged particle beam, the charged particle beam illuminates the dummy pattern to produce at least one dummy beam propagating downstream of the measurement mark. The detection step comprises detecting a distribution of beam blur of the beamlet attributable to a space-charge effect resulting from the dummy beam.
The method can further include the step of disposing a second beam-limiting diaphragm downstream of the first beam-limiting diaphragm. The second beam-limiting diaphragm comprises a respective diaphragm plate defining a respective beam-limiting aperture, wherein the respective diaphragm plate blocks charged particles scattered by the charged-particle scattering membrane. Thus, measurement contrast is further enhanced.
In another method embodiment, a beam-transmitting measurement mark is defined at an object plane, and a knife-edged reference mark is defined, at an image plane, as a corresponding through-hole in a charged-particle-scattering membrane. The measurement mark is illuminated with a charged particle beam to form a charged-particle beamlet propagating downstream of the measurement mark toward the reference mark. The beamlet is projected onto the reference mark while scanning the beamlet over a knife-edge of the reference mark to produce non-scattered charged particles transmitted through the through-hole and forward-scattered charged particles transmitted through the membrane. The non-scattered and forward-scattered charged particles propagate downstream of the reference mark. Using a detector situated downstream of the reference mark, a beam current of charged particles propagating downstream of the reference mark is detected. Between the reference mark and the detector, the non-scattered charged particles are selectively allowed to propagate to the detector while propagation of most of the forward-scattered charged particles to the detector is blocked. With this embodiment, beam blur can be measured at high accuracy without adversely affecting measurement contrast. The measurement of beam blur can be accomplished, for example, by determining the distance over which a rise (12% to 88%) of the differentiation waveform of the detected beam current is observed.
The knife-edged reference mark can be defined as a corresponding aperture defined in a charged-particle-scattering membrane. Use of a thin-film membrane (made of silicon with a thickness of 2 xcexcm, for example) facilitates formation of a high-quality knife-edge having a desired straightness and absence of edge roughness.
In the method embodiment summarized above, the excluding step can comprise defining a beam-limiting aperture in a beam-limiting aperture plate, and disposing the beam-limiting aperture plate between the reference mark and the detector such that the non-scattered charged particles pass through the beam-limiting aperture and most of the forward-scattered charged particles are blocked by the aperture plate. The projecting step can be performed using a projection-lens system comprising a first projection lens and a second projection lens. In this instance the beam-limiting aperture desirably has a diameter such that an axial angle of the beam-limiting aperture as measured at the knife-edge is slightly greater than an angle of convergence of the beamlet at the substrate (i.e., at the image plane).
For example, if the acceleration voltage of the charged particle beam illuminating the measurement mark is 100 kV and the angle of convergence of the beamlet 6 mrad, then the axial angle of the beam-limiting aperture as measured at the knife-edge is 6 to 10 mrad. With such a configuration, 100% of the non-scattered charged particles electrons pass through the beam-limiting aperture, whereas only 0.1% or less of the forward-scattered electrons pass through, allowing measurements to be obtained at maximal contrast.
Another aspect of the invention is set forth in the context of a CPB microlithography apparatus in which a reticle, defining a pattern to be transferred to a sensitive substrate, is irradiated with a charged-particle illumination beam to form a charged-particle patterned beam. The patterned beam is formed by passage of the illumination beam through an illuminated portion of the reticle. The patterned beam carries an aerial image of the illuminated portion of the reticle; the aerial image is projected onto a sensitized surface of a substrate. Specifically, in such an apparatus, a device is provided for evaluating imaging performance of the apparatus. An embodiment of such a device comprises a beam-transmitting measurement mark situated at an object plane of the CPB microlithography apparatus, and a knife-edged reference mark defined at an image plane as a corresponding through-hole in a charged-particle-scattering membrane. An illumination-lens assembly is situated and configured to direct a charged particle beam at the measurement mark so as to form a charged-particle beamlet propagating downstream of the measurement mark toward the reference mark. A projection-lens assembly is situated and configured to project the beamlet onto the reference mark and to scan the beamlet over a knife-edge of the reference mark. Non-scattered charged particles transmitted through the through-hole and forward-scattered charged particles transmitted through the membrane propagate downstream to a beam-limiting diaphragm situated downstream of the reference mark. The beam-limiting diaphragm comprises a diaphragm plate defining a beam-limiting aperture that passes the non-scattered charged particles as the diaphragm plate blocks most of the forward-scattered charged particles. The device also includes a detector situated and configured to detect a beam current of the charged particles propagating downstream of the beam-limiting diaphragm.
The beam-limiting diaphragm can be regarded as a first beam-limiting diaphragm in a device embodiment that includes a second beam-limiting diaphragm situated between the first beam-limiting diaphragm and the detector. The second beam-limiting diaphragm comprises a respective diaphragm plate defining a respective aperture. The respective aperture is configured to pass the non-scattered charged particles as the respective diaphragm plate blocks the forward-scattered charged particles.
Another device embodiment comprises a beam-transmitting measurement mark situated at an object plane of the CPB microlithography apparatus, and a knife-edged reference mark defined at an image plane as a corresponding through-hole in a charged-particle-scattering membrane. The reference mark is situated such that a charged-particle beamlet formed by passage of a charged particle beam through the measurement mark can be scanned over the reference mark to produce non-scattered charged particles passing through the reference mark and forward-scattered charged particles passing through the membrane. A beam-limiting diaphragm is situated downstream of the reference mark. The beam-limiting diaphragm comprises a diaphragm plate defining a beam-limiting aperture that passes the non-scattered charged particles as the diaphragm plate blocks most of the forward-scattered charged particles. A detector is situated downstream of the beam-limiting diaphragm and is configured to detect a beam current of the charged particles propagating downstream of the beam-limiting diaphragm. Connected to the detector is a beam-blur measurement means configured to measure beam blur from detection data obtained by the detector.
The beam-limiting diaphragm can be regarded as a first beam-limiting diaphragm in device embodiments that further comprise a second beam-limiting diaphragm situated between the first beam-limiting diaphragm and the detector. The second beam-limiting diaphragm comprises a respective diaphragm plate that defines a respective aperture. The respective aperture is configured to pass the non-scattered charged particles as the respective diaphragm plate blocks the forward-scattered charged particles.
The beam-limiting diaphragm can be situated 2-20 mm, for example, downstream of the knife-edged reference mark. The detector can be a combination of a photomultiplier and a scintillator, a Faraday cup, or a semiconductor detector, any of which providing high-sensitivity detection of beam blur.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.